Pulverizers are well known for the reduction of the particle size of solid fuel to allow for combustion of the solid fuel in a furnace. A pulverizer employs some combination of impact, attrition and crushing to reduce a solid fuel to a particular particle size. Several types of pulverizer mills can be employed for the pulverization of the solid fuel, for example, coal, to a particulate size appropriate for firing in a furnace. These can include ball-tube mills, impact mills, attrition mills, ball race mills, and ring roll or bowl mills. Most typically, however, bowl mills with integral classification equipment are employed for the pulverization of the solid fuel to allow for transport, drying and direct firing of the pulverized fuel entrained in an air stream.
Bowl mills have a grinding ring carried by a rotating bowl. Fixed position rollers are mounted on roller journal assemblies such that the roll face of the rollers are approximately parallel to the inside surface of the grinding ring and define a very small gap therebetween. Pressure for grinding is applied through springs or hydraulic cylinders on the roller journal to crush solid fuel caught between the roll face of the roller and the grinding ring.
An air stream is typically utilized for drying, classification, and transport of the solid fuel through the pulverizer. The air stream employed is typically a portion of the combustion air referred to as the primary air. The primary air is combustion air first directed through a preheater whereby the combustion air is heated with energy recovered from the flue gas of the furnace. A portion of the primary air is then ducted to the pulverizers. In a bowl mill, the primary air is drawn through beneath the bowl of the bowl mill and up past the roller journal assemblies to collect the pulverized solid fuel. The small particles of solid fuel become entrained in the primary air. The air stream containing the solid fuel then passes through a classifier into the outlet of the pulverizer. After passing through the exhauster, the pulverized fuel can be stored, or more typically, is transported to the furnace by the air stream for direct firing.
For example, U.S. Pat. No. 4,706,900 entitled “Retrofitable Coiled Spring System,” which issued on Nov. 17, 1987 and which is assigned to the same assignee as the present invention illustrates a prior art form of bowl mill using a coiled spring assembly for applying pressure on the roller journal to crush solid fuel caught between the roll face of the roller and the grinding ring. U.S. Pat. No. 4,706,900 discloses both the nature of the construction and the mode of operation of a bowl mill that is suitable for use for purposes of effecting the pulverization of the coal that is used to fuel a coal-fired steam generator.
The journal loading, which dictates the amount of grinding force that the grinding rolls exert on the coal, as mentioned above has been provided to date either through the use of hydraulic systems or through the use of mechanical springs. One such arrangement of mechanical springs can be found depicted, for example, in U.S. Pat. No. 4,706,900. In accord with a showing contained in this U.S. patent, each grinding roll is urged towards the surface of the grinding table by means of an adjustable spring. To this end, a suitable mechanical coiled spring that possesses desired design characteristics is selected; namely, a spring that is capable of urging the grinding roll toward the grinding table surface in such a manner that the grinding roll exerts a predetermined grinding force on the coal disposed on the table, when the coal is of a predetermined depth on the table.
Although the coiled spring assembly constructed in accordance with the teachings of U.S. Pat. No. 4,706,900 has demonstrated to be operative for the purpose for which it has been designed, a need still exists to improve the coiled spring assembly. More specifically, a spring extension cap is presently incorporated on bowl mill type journal spring assembly to cover a spring stud extending from the coiled spring assembly in order to seal the coiled spring assembly. By sealing the exposed end of the spring assembly that is exposed to atmospheric conditions, the extension cap eliminates a differential pressure across the spring assembly so that solid fuel dust, such as coal dust, for example, cannot flow into a bushing assembly having a bushing which allows the spring stud to extend therethrough and to translate axially with respect to the bushing assembly of the coiled spring assembly.
It is desirable to visually inspect an end of the spring stud to determine the amount of spring movement, which indicates relative journal and grinding roll movement. However, when a spring extension cap is in place, the end of the spring stud cannot be visually inspected without removing the extension cap. Therefore, the extension cap must be removed to monitor spring stud movement, which allows solid fuel coal (e.g., coal dust) to flow into the coiled assembly as a result of the differential pressure across the spring assembly and cause premature failure of the bushing or spring stud.
Therefore, there remains a need for an apparatus and method for sealing a coiled spring assembly, which facilitates inspection of an end of a spring stud extending therefrom to determine an amount of spring movement of the coiled spring assembly.